The preparation of glycidyl polyethers of polyhydric phenols, particularly dihydric phenols is well-known. For example, such glycidyl polyethers of dihydric phenols may be prepared by reacting the required proportions of the dihydric phenol and epichlorohydrin in an alkaline medium. The desired alkalinity is obtained by adding basic substances, such as sodium or potassium hydroxide, preferably in stoichiometric excess to the epichlorohydrin. The reaction is preferably accomplished at temperatures within the range of from 50.degree. C. to 150.degree. C. The heating is continued for several hours to effect the reaction and the product is then washed free of salt and base.
By adjusting the relative amounts of the reactants and the process conditions, the resulting glycidyl polyethers will range from liquids to solid epoxy resins. See, for example, U.S. Pat. No. 2,633,458, wherein the preparation of various glycidyl polyethers of dihydric phenols is described. Polyether A is a liquid, whereas Polyether E is a solid epoxy resin. This technique for the preparation of solid epoxy resin requires constant process and control changes to accommodate the great variety of molecular weight ranges demanded by the various users of epoxy resins. Accordingly, there is a growing trend to utilize a so-called "fusion" technique wherein a liquid polyether similar to Polyether A of U.S. Pat. No. 2,633,458 is prepared. Then, using this liquid epoxy resin as a base resin, solid resins having a wide range of molecular weights may be subsequently prepared by reacting said liquid resin with an appropriate amount of a dihydric phenol in the presence of an etherification catalyst. This fusion process is described in U.S. Pat. No. 3,477,990, U.S. Pat. No. 3,547,881, and U.S. Pat. No. 3,978,027. In some respects; however, solid epoxy resins prepared by this fusion technique do not exhibit the same physical properties as the solid resins prepared by the process described in U.S. Pat. No. 2,633,458, especially in pipe coatings.
The use of tris(hydroxymethyl) aminomethane to inhibit the crystallization of liquid diglycidyl ethers of Bisphenol A is known. See, for example, U.S. Pat. No. 3,477,981. Prepolymers of tris(hydroxymethyl) aminomethane and liquid epoxy resins are also known. See, for example, U.S. Pat. No. 3,607,833. These prepolymers contain a relatively large amount of tris(hydroxymethyl) aminomethane, e.g., from about 5 to 14%.
It has now been found that the physical properties of cured solid epoxy resins prepared via the fusion technique can be significantly improved by reacting the solid fusion epoxy resin with a small amount of tris(hydroxyalkyl) aminomethane, generally less than 1%.